1. Field of Invention
The present invention relates to semiconductor light-emitting devices and, more particularly, to improving the quantum efficiency of III-Nitride light-emitting devices.
2. Description of Related Art
III-Nitride light-emitting devices are based on semiconducting alloys of nitrogen with elements from group III of the periodic table. Examples of such III-Nitride devices include InxAlyGa1-x-yN (0≦x≦1, 0≦y≦1, x+y≦1) based light emitting diodes (LEDs) and laser diodes (LDs). Such III-Nitride light emitting devices are commercially valuable high brightness sources of, for example, ultraviolet, blue, and green light.
InxAlyGa1-x-yN crystals such as those from which InxAlyGa1-x-yN light emitting devices are formed typically adopt the wurtzite crystal structure and are consequently polarized even when not externally biased. The two types of polarization that are significant for III-Nitride light-emitting devices are the spontaneous and the piezoelectric polarizations. The spontaneous polarization arises from differences in the electronegativity of the constituent atoms of the III-Nitride semiconductor layers. The piezoelectric polarization arises from strain applied to the III-Nitride semiconductor layers by, for example, neighboring layers of different composition (and thus having different lattice constants). The total polarization in a III-Nitride semiconductor layer is the sum of the spontaneous polarization and the piezoelectric polarization of that layer. In a case where adjacent layers of III-Nitride semiconductors have different total polarizations, there will be a net polarization at the interface between the layers. The effect of the net polarization is to create sheets of fixed charge in the device at interfaces between III-Nitride layers of different alloy composition, and the magnitude and sign of the fixed sheet of charge is equal and opposite to the net polarization. The effect of the sheets of fixed charge is to create corresponding electric fields within the layers. These electric fields will be referred to herein as “polarization fields.” The signs and magnitudes of the sheet charges and of the electric fields are determined by the crystal orientation, the strain in the layers, and the alloy compositions of the layers.
FIGS. 1A and 1B show conventionally simulated band structure diagrams for a portion of an InxAlyGa1-x-yN light-emitting device including an n-type GaN layer 2 formed over a substrate (not shown), an active region 4 (including barrier layers 6 and quantum well layers 8), a p-type Al0.2Ga0.8N electron confinement layer 10, and a p-type GaN contact layer 12. The various layers illustrated are of wurtzite crystal structure with the c-axis of the crystal substantially perpendicular to the layers and directed from layer 2 toward layer 12. The simulations assume that the light-emitting device is driven at a forward current density of about 200 Amps/centimeter2 (A/cm2). The horizontal axes in FIGS. 1A and 1B represent position in the device in a direction perpendicular to the layers. The interfaces between the layers are indicated by dashed lines. The vertical axes in FIGS. 1A and 1B represent the energy of the conduction band edge Ec and of the valence band edge Ev in the various layers.
In FIG. 1A, the simulated band structure accounts for polarization fields in n-layer 2 and active region 4, but neglects the polarization field in p-type Aly,Ga1-yN electron confinement layer 10. Hence, this simulation is not physically realistic. By comparison to FIG. 1B, however, it will serve to illustrate the importance of the polarization field actually present in confinement layer 10 in prior art devices. The tilting of band edges Ec and Ev in the quantum wells and barrier layers of active region 4 as shown in FIG. 1A has been previously recognized to degrade the performance of such III-Nitride light-emitting devices. See, for example, U.S. patent application Ser. No. 09/912,589, titled “Light Emitting Diodes with Graded Composition Active Regions,” assigned to the assignee of the present invention, and incorporated herein by reference. See also International Patent Application WO 01/41224 A2.
In FIG. 1B, the simulated band structure accounts for spontaneous and piezoelectric polarization fields in p-type AlyGa1-y-N electron confinement layer 10 as well as for polarization fields in layer 2 and in active region 4. In particular, this simulation includes a negative sheet charge located at the interface of electron confinement layer 10 and active region 4, and a positive sheet charge located at the interface between confinement layer 10 and contact layer 12.
A comparison of FIGS. 1A and 1B shows that the effect of the sheet charges around AlyGa1-yN electron confinement layer 10 in FIG. 1B is to reduce the magnitude of the potential energy barrier 14 that inhibits electrons from diffusing out of active region 4 into electron confinement layer 10. This reduction in electron confinement due to the polarization field in electron confinement layer 10 causes the quantum efficiency (ratio of photons out to electrons in) of the prior art device illustrated in FIG. 1B to be lower than the quantum efficiency of the hypothetical device (having no polarization field in layer 10) illustrated in FIG. 1A, particularly at high drive current densities.
The difference in quantum efficiencies between the two devices is apparent in FIG. 1C, in which the quantum efficiency 16 of the prior art device of FIG. 1B and the quantum efficiency 18 of the hypothetical device of FIG. 1A are plotted as a function of forward drive current density. These quantum efficiencies were simulated by conventional techniques. As FIG. 1C indicates, at a forward current density of about 400 A/cm2 the hypothetical device of FIG. 1A in which electron barrier layer 10 has no polarization field is three times as efficient as the prior art device of FIG. 1B in which electron barrier layer 10 is polarized.
Since the tilting of the band edges in the active region is substantially the same for the devices of FIGS. 1A and 1B, the drop in quantum efficiency of the prior art device shown in FIG. 1B at high drive current densities must be primarily due to the reduction in height of potential barrier 14 by the polarization field in electron confinement layer 10. The inventors believe that, in contrast to the effects of polarization fields in the active region, the decrease in electron confinement and hence in quantum efficiency due to the polarization field in the electron confinement layer has not been previously recognized. Moreover, polarization fields in n-type hole confinement layers may similarly reduce hole confinement and hence quantum efficiency. The inventors believe that the latter problem also has gone unrecognized.
What is needed is a III-Nitride light-emitting device having improved confinement of electrons and holes to its active region.